The physics core is responsible for the maintenance of the positron emission tomograph (PET) and its associated computer systems, the development of image reconstruction, display and analysis software, and the development of methods to improve the quality and quantitative accuracy of the image data. The current tomograph is a General Electric Advance system. It includes 18 rings of detectors to produce 35 image planes. The tomograph data processing systems utilize dedicated hardware for data acquisition and reconstruction (including a 20-cpu array for reconstruction). System control, image display, and all user interfaces are provided by four SUN UNIX workstations. The facility includes a central Compaq Alpha 2100/375 UNIX server and a Macintosh OsX server for software development and image processing. An additional Macintosh server supports a secure database for the research projects. The tomograph provides a limiting resolution of 4 mm, a slice thickness of 5 mm, and a sensitivity of 210 kcps/muCi/cc in 2D mode and 1160 kcps/muCi/cc in 3D mode. The machine can produce true coincidence count rates of greater than 600,000 events/second. The field of view is 55 cm with a patient tunnel diameter of 60 cm. The Core support is divided into two sections: 1) tasks that are unique to this P01 grant and 2) tasks that are common for all research studies in the PET facility. The majority of the Core support is of the second type and is provided through cost centers that have been setup to operate the PET system. All users of the PET facility are charged an hourly rate to support the costs of performing the scans, maintaining the tomograph, implementing general improvements of the tomograph, and providing special tools for research studies. Under the cost center approach, all users of the facility contribute to the costs of operation in direct proportion to their actual use requirements. Improvements under development include software tools for image registration (rigid and non-rigid body algorithms), kinetic modeling, and general image analysis. Additional projects include testing improved scatter and attenuation correction algorithms in developed in cooperation with the tomograph vendor and optimization of imaging protocols.